A wind turbine is a rotating machine which converts the kinetic energy from the wind into mechanical energy that is then converted to electricity. Wind turbines have been developed for land based installations as well as offshore installations. The land based wind turbines are fixed to the ground and located in windy areas. There are vertical axis wind turbines that have the main rotor shaft arranged vertically and horizontal axis wind turbines that have a horizontal rotor shaft that is pointed into the wind. Horizontal axis wind turbines generally have a tower and an electrical generator coupled to the top of the tower. The generator may be coupled directly or via a gearbox to the hub assembly and turbine blades.
Wind turbines have also been used for offshore applications. Single tower offshore systems are mounted into the sea bed and limited to shallow water depths up to 30 meters. If the turbine tower is mounted on a wider base, such as a lattice structure, this shallow depth requirement can be extended to 50 m. In deeper water, only floating systems are expected to be economically feasible. The drawback of shallow water systems is that the water is typically only shallow close to shore. Thus, wind turbines close to shore can block the shore view and create navigational obstructions and potential hazards for water vessels and aircraft.
Currently, there are a number of concepts for offshore floating wind turbine platforms being developed. Generally, these fall into three main categories: Spars; Tension Leg Platforms (TLP's); and semi-submersible/hybrid systems. Examples of floating wind turbine platforms include the Statoil Norsk-Hydro Hywind spar, (FIG. 1), the Blue H TLP recent prototype (FIG. 2), the SWAY spar/TLP hybrid (FIG. 3), the Force Technology WindSea semi submersible (FIG. 4) and the Trifloater semi submersible (FIG. 5). With reference to FIG. 1, spars are elongated structures that are weighted with significant ballast at the bottom of the structure and buoyant tanks near the waterline. For stability purposes, the center of gravity must be lower than the center of buoyancy. This will insure that the spar will float upright. The spar is moored to the sea floor with a number of lines that hold the spar in place. In general terms, spar type structures have better heave performance than semi-submersibles due to the spar's deep draft and reduced response to vertical wave exciting forces. However, they also have more pitch and roll motions than the other systems, since the water plane area which contributes to stability is reduced in this design.
With reference to FIG. 2, TLPs have vertically tensioned cables or steel pipes that connect the floater directly to the sea bed. There is no requirement for a low center of gravity for stability, except during the installation phase, when buoyancy modules can be temporarily added to provide sufficient stability The TLPs have very good heave and angular motions, but the complexity and cost of the mooring installation, the change in tendon tension due to tidal variations, and the structural frequency coupling between the tower and the mooring system, are three major hurdles for TLP systems.
When comparing different types of offshore wind turbine structures, wave and wind induced motions are not the only elements of performance to consider. Economics play a significant role. It is therefore important to carefully study the fabrication, installation, commissioning/decommissioning costs and ease of access for maintenance methodologies. Semi-submersible concepts with a shallow draft and good stability in operational and transit conditions are significantly cheaper to tow out, install and commission/decommission than spars, due to their draft, and TLPs, due to their low stability before tendon connection.